Controlled on/off switches for controlling the switching edges of current delivered from a power transistor are currently formed of discrete components including a bipolar transistor, which is driven by a constant voltage edge when turned on and off. This rather crude technique has a disadvantage in that the propagation times are long and the switching edges of the output current are dependent on the transistor beta.
An embodiment of a prior art switch adapted to connect the battery to the battery charger in a "soft" manner, is shown in FIG. 1. The operation of the circuit device of FIG. 1, and its applications, will now be described in better detail. The battery can be modeled by a voltage generator whose value varies typically from a few hundred milliVolts, in the exhausted condition, to 5-10V when fully charged, and a series resistor of very low value (100 to 200 milliOhms). The battery charger. can be represented schematically by a current generator 2 which is saturated at a predetermined voltage. In this particular instance, the values of current and voltage would be 1.2 A and 16V, respectively.
The device depicted in FIG. 1 complete of its served circuits, operates in either of two modes: a start-up mode and a PWM mode. It enters the start-up mode when the battery is exhausted, and is held there for a few tenths of a second, enough time to have the battery charged to an intermediate level (about 3 Volts). The transistor P1 is off, and the voltage regulator 4 is activated to limit the current at 180 mA. During this initial phase where the battery is in the exhausted condition, it is important that the battery be charged with a smaller current than that supplied by the battery charger, to avoid damaging the battery.
Upon a battery voltage being reached at which all the circuitry to be powered by the battery can begin to operate (for example, 3 Volts in this case), the start-up regulator 4 is turned off and the device allowed to operate in the PWM mode. During the latter phase, the cellular telephone is able to operate properly, and the battery charging is handled by a microprocessor, outside the device 1 in question, which controls the transistor P1 through its PWM pin. Hence the need to control the battery charging current edges so as to prevent steep switchings from disturbing the communications.
Thus, it can be appreciated from FIG. 1 that this conventional approach has the following disadvantages:
the propagation time of the turn-on command is quite long and dependent on the voltage at the VCH pin, to which the charging current from the generator 2 is applied, and on the base-emitter junction voltage of the transistor P1, i.e., EQU T.sub.ON =R1 * C1 * In(V.sub.C H/V.sub.C H-V.sub.B E); (1) PA1 the current edge is a coarse value, and dependent on the voltage at the pin VCH and the beta of the transistor P1.
The underlying technical problem of this invention is to provide a circuit for controlling the switching edges of a power transistor, which can operate at low propagation times and control the output current with high accuracy, thereby overcoming the aforementioned limitations of the prior art.